1. Field of the Invention
The present invention relates to a reflector that can be suitably used for a reflection type liquid crystal display device that uses ambient light as a light source. More particularly, the present invention relates to a reflector providing desirable reflectance over a wide angle and a particularly high reflectance in an intended range of directions in which light is reflected, and a reflection type liquid crystal display device providing a wide viewing angle and suitable directionality so that a display surface appears bright within a typical range of viewing angle for a display device incorporated in certain devices such as a notebook personal computer.
2. Description of the Related Art
In recent years, a reflection type liquid crystal display device using ambient light as a light source is widely used as a display part of a handy personal computer and the like particularly because of its low power consumption. A reflection type liquid crystal display device has a reflector which reflects incident light coming through the display surface side back to the display surface side so that the user can view a display that is produced according to the arrangement of liquid crystal molecules in a liquid crystal layer.
When a reflector having a flat surface is used for a reflection type liquid crystal display device, the reflector has high reflectance in a particular reflection angle corresponding to an incident angle. However, the range of reflection angle showing high reflectance is narrow, i.e., the viewing angle is narrow. To solve such a problem, there are several attempts so as to obtain good reflectance in wider range of direction, for example, by forming many concave portions or grooves each being a part of a sphere on a reflector surface, or by providing depressions and projections randomly.
FIG. 15 shows a reflector provided with many concave portions being a part of a sphere on a reflector surface. A reflector 71 shown in this figure is formed as follows. On a substrate 72 made of a glass or the like, a flat-plate resin base material 73 (a base material for a reflector) made of a photosensitive resin layer or the like is provided. On a surface of the base material 72, many concave portions 74 whose inner surfaces being a part of a sphere are formed continuously so as to overlap each other. A reflection film 75 made of a thin film of aluminum, silver or the like is deposited or printed on the concave portions 74.
The concave portions 74 are formed with random depth in a range of 0.1 μm to 3 μm and are arranged randomly with the pitch between adjacent concave portions ranging from 5 μm to 50 μm. An inner surface of each of the concave portions 74 is a curved surface which is a part of a single sphere.
The term “depth of a concave portion” as used herein means the distance from the reflector surface to the bottom of the concave portion, and the term “pitch of concave portions” as used herein means the distance between the center of a concave portion (which has a circular shape as viewed in a plan view) and the center of an adjacent concave portion.
The reflector 71 has a reflection property shown as β in a comparative example of FIG. 7 or FIG. 12. Each of FIG. 7 and FIG. 12 is a graph showing a reflection property when the incident angle is 30°, wherein the vertical axis is reflectance (reflection intensity), and the horizontal axis is the reflection angle. The term “incident angle” as used herein means an angle ω0 between incident light J and a normal line H extending to the surface of the reflector 71 as shown in FIG. 16. Likewise, the term “reflection angle” as used herein means an angle ω between the normal line H and reflection light K on a plane including the incident light J and the normal line H. As β shown in the comparative example of FIG. 7 or FIG. 12, the reflector 71 shows relatively good reflectance, which is in a range of 15°≦ω≦45° centered about the reflection angle 30°.
The conventional reflector 71 described above enables one to obtain relatively good reflectance over a relatively wide angle due to the concave portions. However as β shown in the comparative example of FIG. 7 or FIG. 12, the relatively higher reflection intensity peaks at the reflection angles 15° and 45°, which appear symmetrical with the reflection angle 30° being an axis of symmetry.
Nevertheless, a display device incorporated in devices such as notebook personal computers, in which a display surface is inclined during its use, is generally viewed from near the direction normal to the display surface as shown in FIG. 17 even though it may vary depending on a degree of inclination of the display surface or a position of the light source. FIG. 17 shows a notebook personal computer having a main body 81 and a cover portion 82, illustrating a situation in which the computer is used. In FIG. 17, P represents a direction normal to a conventional display device 83, Q incident light, ω0 an incident angle (e.g., 30°), R1 reflection light whose reflection angle ω is the same as the incident angle ω0, R2 reflection light whose reflection angle ω is smaller than the incident angle ω0, and R3 reflection light whose reflection angle ω is greater than the incident angle ω0.
As seen in FIG. 17, directions in which a user usually looks at the display device 83 are concentrated in a range of the direction of the reflection light R2 near the normal line P as opposed to a range of the reflection light R3 in which the user has to look up at the display device 83 from a lower direction making it more difficult to see it. Therefore, for convenience of the users, it is desirable to secure a wide viewing angle while enhancing reflectance in the direction in which the reflection angle is smaller than reflection light.
To the contrary, a display device on a horizontal surface such as a table-type game machine is generally looked at from a direction near parallel to the surface as shown in FIG. 18. FIG. 18 shows a display device 85 provided horizontally on a table 84, illustrating a situation in which the device is used. In FIG. 18, W represents a direction normal to the display device 85, S incident light, ω0 an incident angle (e.g., 30°), T1 reflection light whose reflection angle ω is the same as the incident angle ω0, T2 reflection light whose reflection angle ω is smaller than the incident angle ω0, and T3 reflection light whose reflection angle ω is greater than the incident angle ω0.
As seen in FIG. 18, directions in which a user usually looks at the display device 85 are concentrated in a range of the direction of the reflection light T3 whose reflection angle is greater than reflection light T1. Meanwhile, the reflection light T2 is in the range of the direction where a user has to look into the display device, thus making it awkward to see it. Accordingly, for convenience of users, it is desirable to obtain a wide viewing angle and particularly high reflectance in a range of directions in which the reflection angle is greater than the incident angle.